1. Field of the Invention
The present invention relates to a sheet binder, and a cover member used when sheets are bound.
2. Related Background Art
In the past, example of sheet binders for binding a plurality of sheets by taking advantage of fusion and solidification of an adhesive, there has been proposed a sheet binder wherein an adhesive is applied onto an inner surface of the back of a cover member and then a plurality of sheets to be bound are inserted inside of the cover member, whereby the sheets are bound together with the cover by fusing the adhesive by application of heat.
Such a conventional sheet binder comprised nichrome wires acting as a heating means for fusing the adhesive, a heat generating means such as a sheet-like heat generating body, a heat plate for supporting the heat generating body, holding the cover member and transmitting the heat from the heat generating body to the cover member, and an insulator for electrically insulating the heat generating means from the heat plate.
However, in the above-mentioned conventional sheet binder, as the size of the sheet to be bound was increased and as the thickness of the sheet bundle to be bound was increased, the heat plate had to be so designed that its surface area, its strength for holding the cover and its thickness also were more increased. As a result, the heat capacity of the heat plate was also increased, and it took a long time to attain a predetermined temperature (about 150.degree. C.) required for fusing the adhesive.
Thus, if the conventional heating means such as a surface heater was used, since the productivity of the initial sheet binding operation was extremely reduced and the heating value (heat amount) of the heat generating body became great, there arose a problem that an additional protection device must be provided for protection purpose so that the binder becomes bulky due to the larger capacity of the electric power source.
Further, since the heat fusible adhesive was heated via the cover member, the heat conductivity was worsened and cover materials having a poor heat resistance could not be used. In addition, if a cover member made of normal paper was used, since the heat conductivity thereof was changed in accordance with its thickness, in order to compensate for the change in heat conductivity in terms of safety, the heating time had to be longer.